hi linux expert
how to use man command for get help about "extended regular expression"?
many thanks
samad
man grep has a section on the e option, ERE or POSIX extended regular expressions. There is also perl regex which some linux systems can invoke. This leads to confusion for new folks.
To get a real background you need to to do a quick read outside of the man application. This one explains with examples the differences for ERE vs. plain (regular):
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man 7 regex fully explains "modern" (extended) regular expressions and how they differ from "basic" REs.